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elizabeth  ·  1735 days ago  ·  link  ·    ·  parent  ·  post: Space fans of Hubski - Why don't moons ever have moons of their own?

Maybe moons are too small to have a big enough gravitational force?

I know some big moons can have rings : https://en.wikipedia.org/wiki/Rings_of_Rhea

How big does a rock that gravitates around a moon has to be, before it's called a "moon" ?